1. Field of the Invention
The present invention relates to a powertrain, and more particularly, to a powertrain for a four-speed automatic transmission used in vehicles, which can accomplish four forward speeds and one reverse speed with three planetary gearsets and fourth friction elements.
2. Description of the Related Arts
Generally, automatic transmission systems for vehicles comprise a transmission control unit (TCU) which automatically controls shift ratios according to changes in a running condition of the vehicle.
The above-described TCU controls a plurality of friction elements provided in a powertrain to either operative or inoperative states to select one of the three essential elements of the planetary gearset (a sun gear, a ring gear, and a planet carrier) to be an input element, another a reaction element, and the last an output element, thereby controlling the number of output revolutions.
In a conventional powertrain for a 4-speed automatic transmission, to drive the vehicle in fourth speed overdrive, there is a need for at least one element which rotates at a higher speed than that of a final output element. This results in power loss. In addition, the large number of friction elements results in a heavy and large-size automatic transmission.